The influence of boric acid, at concentrations of 0.1 and 0.5 M, on anodizing Al 2024-T3 at constant current density in 0.5 M sulphuric acid electrolyte, has been investigated. From the voltage time responses and oxide thicknesses measurements after 30 min anodizing, addition of boric acid has negligible effects on the general anodizing behavior of the alloy. However, in the initial stages of anodizing, the presence of boric acid in the electrolyte strongly reduces the occurrence of secondary electrochemical reactions, consuming electrical charge, consequently increasing the initial efficiency for anodic oxide formation. The effect of boric acid in the initial stages of anodizing the alloy is attributed mainly to the inhibition of oxygen evolution on CuAl2 particles by precipitation of partially or, totally dehydrated boron oxide.

Key words: aluminum alloy, anodic films, anodizing, boric acid

INTRODUCTION

As a result of the widespread application of aluminum alloys there is considerable interest in alloy pre-treatment and protective coating [1]. Concerning copper containing aluminum alloys, the basis for a major group of structural materials, the methods developed to protect the alloy are anodizing and conversion coating . Both protection treatments involve the use of chromic acid solution, which has effectively provided corrosion protection [2]. However, chromate species have become the subject of increasing environmental regulation due to their toxic and carcinogenic properties. Thus, alternative anodizing electrolytes for protecting this alloy material have become important.

An alternative electrolyte for anodizing the Al-2024 is a mixture of sulphuric-boric acids [3]. Considering the industrial use of the electrolyte, there is little information about the anodic behavior of aluminum-copper containing alloys in the particular anodizing solution.

Regarding the information above, in this work high resolution SEM together with EDX have been employed to study the anodizing behavior of a commercial aluminum alloy, Al-2024, in different mixtures of sulphuric-boric solutions. The results are compared with those obtained on anodizing Al-99.7 wt % in order to understand the role of the alloying elements during anodic oxidation.

EXPERIMENTAL

Specimens of Al 2024-T3 and Al-99.7 wt % of dimensions 40 x 10 x 1 mm were degreased at 84ºC in ethylene vapour for 5 minutes and etching in a solution of Smut-GO # 4 for 5 minutes at 20ºC, which was provided by the Chilean Aircraft Industry (ENAER). After rinsing in deionized water, the specimens were anodized at different constant current densities, 5,and 10 mA cm-2, for 30 minutes, in mixtures of sulphuric and boric acids of different composition.

The surface morphology of the specimens after surface treatment was examined in a JEOL 5410 scanning electron microscope, equipped with energy dispersive X-ray (EDX) analysis facilities. The thickness of selected anodic films was determined from observations of fractured film sections in the SEM.

Potentiodynamic polarization experiments were carried out using a WENKING POS-73 potentioscan on three electrodes, namely Cu 99,999 wt %, Al both of 99.97 wt % purity, and Al-2024 T-3, in the same anodizing electrolytes. In each experiment, the working electrode was mechanically polished with 800 grit paper, ultrasonically cleaned and rinsed with distilled water. Prior to measurements, solutions were deaerated with N2 and specimens were allowed to reach a steady potential. A conventional electrochemical cell, with three compartments, was employed, using a platinum counter electrode and a saturated sulphate- mercurious sulphate electrode.

RESULTS

Surface morphology

Figures 1 and 2 show SEM micrographs, revealing the surface morphology of Al-99.97 wt % and Al-2024 T-3. As expected, for the aluminum of high purity a relatively smooth surface is evident. In contrast, for the Al-2024 light regions, of dimensions between 10 and 70 m m, with a population density of about 8.4 x 109 m-2, decorate the surface. These regions, of relatively uniform distribution, are associated with second phase material, probably with intermetallic particles of CuAl2, since EDAX analyses of such surface regions revealed local high copper content of about 50 wt%. Far from the light features, EDX analysis revealed relatively uniform content of alloying elements.

Figures 3, and 4 show the typical voltage-time behavior during galvanostatic anodizing of Al-99.97 wt% at different current densities, in 0.5 M sulphuric acid, in the absence and presence of different boric acid contents. In all cases an initial sudden increase in the voltage to 6 V is observed, which is associated with the air-formed oxide and/or with the chemically formed oxide during the surface treatment. With further anodizing, the voltage increases linearly with time, at a rate depending on the current density employed. After reaching the maximum voltage, the voltage decreases gradually with time and eventually achieves a steady state voltage, of values depending also on the current density employed. In the initial, approximately linearly rising voltage periods of the voltage-time behavior, where barrier oxide predominates, the voltage rises at rates of 72-78 and 145-152 V min-1 for anodizing at 5 and 10 mA cm-2 respectively. The rates are considerably less than 147.3 and 294.6 V min-1, which are the expected values for anodizing at 5 and 10 mA cm-2 in 0.05 M ammonium adipate solution at 298 K, where barrier oxide formation occurs at 100% current efficiency [4]. Thus, it is evident that for anodizing at the different current densities in 0.5 M sulphuric acid and the mixtures of sulphuric-boric acids solutions the barrier oxide growth is accompanied by a considerable loss of Al+3 ions from the onset of anodic oxidation. From comparison of the voltage-time response depicted in Figs.3 and 4 no changes result from addition of boric acid into the electrolyte.

Fig.5 and 6 illustrates the voltage-time behavior of the Al 2024-T3 at different current densities. As observed for anodizing aluminum of high purity, from both voltage-time responses an initial sudden increase in the voltage is disclosed, but to 3.4 V, implying possibly a pre-existing film. thinner than on aluminum.. The voltage then increases linearly with time up to 5-7 V, at rates (s1 in table 1) depending on the particular anodizing condition, remaining at this voltage for a time (p in table 1) dependent on the solution composition, and current density employed. During the period at 5-7 V, abundant gas generation, associated with oxygen evolution, was visually observed. After such a period , the voltage starts increasing at a rate (s2 in table 1) depending again on the selected conditions for anodizing, to decrease progressively to reach a steady state voltage.

From comparison of the anodizing data for superpure aluminum with that for Al-2024, the rates of voltage rise for anodizing the alloy ( s1 and s2) are lower, indicating that consuming charge processes proceed during anodizing the alloy. For instance, in the initial stages of anodizing the alloy in 0.5 M sulphuric acid, the rate of voltage rise (s1) is 34 V min-1, which represents approximately 50 % of the current efficiency for anodizing aluminum of high purity. Further, the second V-t slope (s2), after the period at 5-7 V, is even lower, 10 V min-1, representing 13% of the current efficiency for anodizing superpure aluminum.

From Figs.5 and 6 and the data in Table 1, it can be observed that additions of boric acid increases the first and second rates of voltage rise. Although for anodizing at 5 mA cm-2 small additions of boric acid do not affect the initial rate of voltage rise, with increasing the boric acid content an augment of the first V-t slope from 34 to 44 V min-1 is observed. For anodizing at 10 mA cm-2, the effect of boric acid in increasing s1 and s2 during anodizing the alloy is more marked. Thus, the initial rate of voltage rise of 72 V min-1, obtained during anodizing in 0.5 M sulphuric acid, reaches values of 87 and 128 V min-1 in the presence of 0.1 and 0.5 M of boric acid respectively.

Generally, anodizing of Al 2024-T3 produced great amounts of oxygen after the initial rate of voltage rise, with an associated plateau at 5-7 V. The high rate of production of oxygen is probably associated with the second phase CuAl2 present in the alloy material (Fig.2), as discussed later From Figs. 5 and 6 and Table 1 it can be seen that the presence of boric acid in the anodizing electrolyte reduces the period at 5-7 V.

Influence of boric acid on the electrochemical behavior of Al, Cu and Al-2024 T-3.

In order to understand the effect of boric acid during anodizing Al 2024 T3, electrochemical measurements were carried out on high purity aluminum, copper and Al-2024 in two electrolytes, 0.5 M sulphuric acid and, 0.5 M sulphuric with additions of 0.5 M H3BO3. For aluminum of high purity the potentiodynamic curves between -0.9 and 1 V are depicted in Fig.7. As observed, the current response in the absence and presence of boric acid is similar. From the I-E curves, an initial relatively rapid increase in the current density above -0.84 V to a maximum, followed by a region of approximately constant current density, associated with thickening of the alumina film. For the Al-2024 alloy however (Fig.8), the current response in the same potential range is different from that of aluminum. For the alloy, anodic oxidation of aluminum starts at -0.6 V, indicating a displacement with respect to that of aluminum of high purity of about 0.26 V in the anodic direction. This potential displacement is mainly associated with the presence of copper in the alloy, which, as expected, shifts the open circuit potential of aluminum in the anodic direction [5,6]. After the initial increase of the current, the electrode tends to become passivated at about -0.35 V, but a second sudden increase in the current at about -0.25 V, depicting a complex current peak between -0.25 and 0.15 V, interrupts passivation. Such a current peak is possibly related to secondary electrochemical reactions, which consuming charge, delay oxidation of aluminum.. However, as the potential increases, beyond 0.15 V the electrode reaches a steady-state current implying that thickening of the alumina film becomes the prevalent electrochemical reaction. From comparison of the potentiodynamic curves of copper (Fig.9) with that of the Al-2024 T-3 electrode, the secondary electrochemical processes proceeding during oxidation of the alloy appear related to copper oxidation and oxygen evolution. This, considering that such processes proceed at the same potential where copper oxidation and oxygen evolution, visually observed, occurs during oxidation of copper.

From Fig.8, it is interesting that additions of boric acid changes the current response of the Al-2024, decreasing the charge involving copper oxidation and oxygen evolution. This reduction in charge for the alloy oxidation between 0.25 and 0.15 V could be related to the slight displacement of the potential for copper oxidation, which is observed when the copper electrode is anodically polarised in the presence of boric acid (Fig.9). The possible manner that boric acid influences the occurrence of secondary electrochemical processes in the initial stages of anodizing is discussed later.

The thickness of alumina films formed for 30 minutes at 10 mA cm-2, determined by SEM examination of fractured sections of films formed on Al-2024 T 3, are similar in the three electrolytes, of about 7 m m. Further, the oxide morphology is similar in the absence and presence of boric acid. This suggest that the effect of boric acid in the initial stages of anodizing has no major influence in the general anodizing behavior of the alloy. This is in agreement with previous results [7], where negligible influence of boric acid for the anodic oxidation of Al-7475 and 2024 is reported..

From comparison of the voltage time responses, boric acid has no influence on the response of aluminum of high purity, but a major influence for the Al-2024 alloy. As discussed shortly, the decrease in efficiency with progress of anodizing, which is found for the aluminum alloy in the initial period of anodizing, is probably associated with the generation of oxygen just above the intermetallic regions, mainly CuAl2, as revealed by SEM and EDX analysis of the alloy surface.

Binary aluminum alloys containing high concentrations of non-valve-metal can only support anodic films of very limited thickness, contrary to binary alloys containing a valve metal [8, 9]. For aluminum alloys containing non valve metals, the formation of semiconducting oxide at the alloy/film interface, by oxidation of either the relatively concentrated bulk alloy or, for less concentrated alloys, clusters rich in alloying element within an enriched alloy layer [10], may result in the production of oxygen just above the semiconducting region [11]. For dilute and concentrated Al-Cu alloys, containing 1 and 2 at % Cu, enrichment of a 2 nm thick alloy layer to about 40 % is necessary prior to oxidation of copper [12,13]. The copper concentration in CuAl2 particles, 33 at % is already high and any required enrichment during anodizing must be achieved very rapidly, through oxidation of a few nm of alloy during the surface treatment. Thus, oxygen is evolved from the CuAl2 particles at the commencement of anodizing, in the presence of an extremely thin layer of alumina, which is really disrupted by oxygen production.

It is clear, from the potentiodynamic polarization study, that the primary role of relatively high boric acid content in the anodizing electrolyte is to limit oxygen evolution, primarily in the early stages of anodizing. This increases the initial rate of voltage rise and decreases the period at 5-7 V. The inhibition of oxygen evolution is attributed to the local precipitation of B2O3 above surface regions of local high copper content associated with CuAl2 particles. The manner that precipitation of boron oxide occurs is thought to be associated with an increase of the local pH above copper rich regions. O2 evolution in acid medium, which proceeds according to the following reaction: 2H2O Û O2 + 4H+ + 4e, will increase the acidity of such regions locally, promoting the partial or, total dehydration of B(OH)3 molecules at the solution-intermetallic particle interface. Dehydration of B(OH)3 units may be enhanced by the high electric field at the commencement of anodizing and the local temperature rise.

The influence of boric acid on the formation of anodic alumina and of oxygen within the anodic alumina close to the alloy/film interface, is considered to be insignificant from previous evidence of incorporation of foreign species into anodic alumina. All examples known to the authors of anion species incorporated into anodic alumina from the electrolyte reveal slower inward migration than O-2 ions and hence, such species do not get close to the alloy/film interface by migration through the film [14]. Further, anion derivatives of boric acid, generated in the electrolyte through loss of H+ ions from the molecule, are considered negligible, because of the high H+ concentration in the solution. In addition, and important boric acid species may access the alloy/film interface at cracks in the film and assist healing by its dehydration, limiting oxygen production at the exposed regions of copper-rich alloy.

CONCLUSIONS

-Additions of boric acid to sulphuric acid solutions increases the efficiency of anodic film growth on the Al-2024 in the early stage of anodizing.

-The beneficial effect of boric acid on the initial anodic efficiency for film formation is considered to be mainly due to the local precipitation of B2O3 at copper sites, which results from dehydration of B(OH)3. The local precipitation of totally or partially dehydrated B2O3 limits the high rate of oxygen evolution and consequently, increases the effective ionic current for oxide formation.

- the effect of boric acid in the initial stages of anodizing has no a major influence in the general anodizing behavior of the alloy.

ACKNOWLEDGEMENTS:

The authors wish to thank FONDECYT (Grant: 1000797) and DICYT, Universidad de Santiago de Chile for financial support. Thanks are also given to Dr.J.E.Foerster for his useful suggestions.